The application of cryopumping has been very effective in pumping hydrogen and its isotopes at very high pumping speeds. Typical applications have included pumping space simulation chambers and high power neutral beam lines. Neutral beam injection line pumps for example pump at speeds of 250,000 l/s. These pumps are cooled with liquid helium and typically are surrounded with liquid nitrogen cooled thermal radiation shields. The operation of these pumps involves initially pumping the vacuum chamber with an auxillary pumping system, e.g., a turbo pump to a pressure at or below 10.sup.-4 torr and then chilling the pumping surfaces to 4-6 degrees K using liquid helium. The system is then put into operation with the material being pumped collected on the cold surfaces as a frozen solid frost like. Pumping can continue indefinitely unless the frost layer becomes excessively thick limiting the thermal conduction to the pumping surface. In many applications the collection of material on the pump is limited to other criteria, e.g., when pumping hydrogen it may be desirable, for safety reasons, to limit the total inventory to a value below the explosive limit inside the vacuum chamber. Also, when using tritium it is usually desirable to limit the total inventory of the radioactive gas. In all cryopumping systems to date, after one of these limits occurs, the pumps would require regeneration. This process involves warming the cryopanels, causing the cryofrost to evaporate. The evaporated frost is then pumped by the auxillary pumping system after which the pumps can be recooled and the pumping cycle repeated.
In many applications the cyclical nature of the pumping cycle cannot be tolerated. In order to pump a chamber continuously, two or more pumps are operated in tandem with the pumps isolated from the process chamber by gate valves. Another technique being developed is to close the liquid nitrogen chevrons of a tandem set of pumps during regeneration and pump the regenerated material with a separate pump which evacuates the volume inside the baffles.
High speed cryopumps also undergo a thermal instability when the process chamber goes above a certain pressure, usually in the range of 10.sup.-3 torr. The cause of this is that the process vacuum serves as cryogenic insulation for the liquid helium cooled pumping surface. Thus as the pressure in the chamber gets too high, the heat conducted through the gas overwhelms the cooling capacity and the panels spontaneously regenerate. Because of this, large cryopumps are usually designed to operate at or below 10.sup.-4 torr. This limitation restricts the throughput of the pump, or necessitates very large pumping speeds in the pump chamber. While this may in many instances be irrelevant it can impact many potential applications. For example, in a tokomak fusion reactor, the throughput of fuel will be in the range of a hundred torr liters per second while the conductance of the vacuum lines and gate valves leading to the pumps may be of the order of 10,000-100,000 l/s. In order to handle the throughput, a typical high speed cryopump operating at 10.sup.-4 torr would require a pumping speed of a million l/s, 10-100 times greater than the conductance.
Cryopumps are not alone in the inability to pump high throughputs at pressures greater than 10.sup.-3 torr as the other potential high speed pumps also develop problems. Turbomolecular pumps and diffussion pumps both lose effective pumping at 10.sup.-3 torr. The most effective pumps in this region of pressure are roots blowers which have significant difficulties in tritium applications due to the need for oil lubrication.
Accordingly, it is an object of the present invention to provide a high throughput continuous cryopump incorporating means for regenerating the cryopumping surface while the pump is in operation. More specifically, it is an object to provide a pump which has the potential of pumping 100 torr - l/s at a speed of 10,000 l/s, which is tritium compatible and can operate continuously. It is a further object of the present invention to provide such a cryopump capable of pumping at high pressures while limiting the cryopumping to a selected surface continuously cleaned by a regenerating device. This cryopumping surface is thermally insulated using a separate vacuum chamber surrounding the cryopumping surface and a metal bellows which connects the cryopumping chamber and its associated cryopumping surface to the pump inlet. It is another object of the present invention to provide such a high throughput cryopump which includes a secondary chamber which receives frost scraped or otherwise thermally removed from the cryopumping surface whereby this frost can be pumped at a higher pressure away from the cryopumping surface after removal.
Other objects and advantages of the present invention will be recognized upon reading the detailed description together with the drawings described as follows.